Japanese Patent Application Publication 2003-079059 discloses a method for charging an assembled battery that is composed of a plurality of lithium secondary cells, such as lithium-ion cells, which are connected to each other. In this method, the current terminal voltage of each lithium secondary cell is compared with a specific full-charge voltage value. When the terminal voltage of at least one of the lithium secondary cells has reached the full-charge voltage value, the charging current starts to be gradually reduced so that other cells can be gradually charged to fully-charged state while the terminal voltage of the fully-charged cell is maintained at the full-charge voltage value.